Radio-frequency vacuum tube voltmeter



May 20, 1952 R. H. DlcKE RADIO-FREQUENCY VACUUM TUBE VOLTMETER Filed July 9, 1945 ATTORN EY Patented May 2), 1952 UNITED freni rice mesne assignments, to the United States of America as represented by the Secretary of War Application July 9, 1945, serial No. 604,009

' 12 claims'. (c1. 1v1-95) This invention relates generally to an electrical circuit and more specincally to radio freqency measuring apparatus.

In general a radio frequency transmission line serves to convey radio frequency energy from one point to another. The hollow wave guide is one form of transmission line wherein energy is guided or conducted from one point to the other. In the coaxial transmission line, current incidentto the transfer of energy flows on the inside of the outer conductor and on the outside of the inner conductor. Y

Energy may be thought of as being transferred by an electromagnetic wave, ccnned within the wave guide or coaxial line. The traveling wave may be considered in regard to its voltage characteristics or its current characteristics. In either case the instantaneous value of current or voltage along the line is not necessarily constant and may vary along the line.

On reaching the terminating end the traveling wave is absorbed if the terminating impedance is equal to the characteristic impedance of the line or reflected if such is not the case. When the terminating impedance is not equal to the characteristic impedance, the reflected wave returns back along the line Aand alters the characteristics of the initial traveling wave. When waves travel on a line in both directions simultaneously, the resultant distribution of voltage along the line is called the voltage standing wave, and the distribution of current is called the current standing wave. The voltage and current magnitudes vary with position along the line with alternate maxima'and minima. Each maxima or minima is' one transmission line wavelength from the succeeding one. The standing-wave ratio is defined asv the ratio of the voltage maximum to the voltage minimum or the current maximum to the currentjminimum. Y

The transfer of energy through a wave guide or coaxial cable may be impeded or attenuated by dielectric losses within the structure, and also by` resistive losses resulting from the skin effect on the inside surfaces of the structure.v l

When radio frequency energy is radiated from awave guide, an antenna, or a dipole, the radiation pattern is a plot of the eld intensities at various positions at the same distance about the radiating element. Y A Y l One object of this inventionisto provide Ameans for measuring the standing wave 4ratio along a transmission line. Another object is toprovide meansA for measuring the losses incident with the transfer of energy in a transmission line. A fur- 2 ther object is to provide means for measuring the radiation pattern of a radio frequency radiating element.

Other objects, features and advantages of this invention will suggest themselves to those skilled in the art and will become apparent from the following description of the invention taken in connection with the accompanying drawing in which:

Fig. 1 shows a circuit embodying the principles of this invention, the transmission line 62 being shown partly in cross-section, and partly in elevation; and

Fig. 2 shows a network to be described later.

Referring to Fig.11 there is shown a radio frequency oscillator I0 modulated by an audio oscillator I2, the output of which is applied to oscillator I0 through a phase shifter II. The modulated carrier appears at output terminal I3 and serves as a reference power input to a transmission line 62 connected to that terminal. A bolometer bridge I4 is used to detect a portion of the audio frequency component of the energy on the line. The bolometer bridge consists of two separate Littlefuse probes I5 and IS connected in a series through a variable dropping resistor I9 between a source of positive potential and ground. .As shown in Fig. 1, the probe I5 is placed within the input end of line 52 and the probe I3 is'v placed withinV the output end of line 62. Across the' two probes I5 and IIS are the two end connections of balancing potentiometer I1 and by-pass condenser I8. The center tap connection of balancing potentiometer I'I is connected to the common connection between the two probes I5 and I6 and the output of the bolometer bridge is taken from this connection. TheV output is fed, in turn, through audio amplier 20 and variable attenuator 2`I. The variable attenuator may be of the ordinary resistive type. Twin T network 28 shown in Fig. 2 consists of two resistors 22 and 23 connected between input termina19 and output terminal 6I and a capacitor 25 connected between the common connection of the two resistors to ground. Two capacitors 26 and 2lY are also connected in series between' the two terminals and 6I with a resistor Z4 tied from a' common connection between the two condensers to ground. Input terminal 50 is connected to the plate circuit while output terminal 6I is connected into the grid circuit of a stage of the audio amplier 20. The amplified audio signal is injected through couplingV capacitor 29 to grid 3o of mixer tube3I in lock-in amplifier 55. Into suppressor grid 32 of .mixer 3| and through coupling capacitor 33 is injected the same audio frequency as was used to modulate the radio frequency carrier. Anode 34 of mixer 3| is returned to a suitable positive potential, B+, through load resistor 35. Ordinary cathode bias on cathode 39 for mixer 3| is provided by a parallel-series combination consisting of resistors 36 and 31 and condenser 38. Grid 30 is returned through resistor 4D to the common connection between resistors 36 and 31 and condenser 38. Resistor 4| to ground acts as a load resistor for suppressor grid 32 of tube 3|. Capacitors 42 and 43 by-pass the A.C. variations of anode 34 and screen grid 54 respectively to ground.

The output of mixer 3| is taken from anode 34 and connected in series through a resistor 41, a parallel combination of resistor 46 and condenser 49, a microammeter 44 and a resistor 45 to ground. A condenser 48 is connected directly from anode 34 to the common connection of microammeter 44 and resistor 45. For changing the range of microammeter 44, a switch U is provided for inserting a shunt 5| across that meter. A suitable positive potential B-lis applied through resistor 65 to screen grid 54 of mixer 3| and through a parallel combination of resistor 52 and zero-adjust resistor 53 to the junction of resistor 45 and microammeter 44. Proper adjustment of zero-adjust resistor 53 will allow correct zero calibration of meter 44.

In the bolometer network I4 shown in Fig. l, the D.C. current through a Littlefuse probe causes a voltage to appear across the probe which is proportional to the resistance of the element. Where the change in power input to the element is low, the change in resistance of the element is proportional to the R.-F. power absorbed. In absorbing a modulated carrier the voltage generated by the probe is therefore proportional to the instantaneous power of the carrier.

For accurate loss measurements a difference measurement must be accurately made. For this reason a bridge employing both Littlefuse probes I5 and I6 is used.

The manner of operation is as follows. A modulated R.F. carrier appearing at terminal I3 serves as a reference power input to transmission line 62 connected to that terminal. To balance the bridge both Littlefuse probes are placed at the same location on the line and potentiometer |1 is adjusted for minimum deection of meter 44.

By placing one Littlefuse probe at the input to the line and the other at the output the audio voltage generated will be proportional to the difference in radio frequency power absorbed by the two elements, or, in other words, will be proportional to the power loss through the line.

The audio voltage generated by the bolometer network is amplified and then mixed in mixer 3| with the same frequency coming directly from the audio oscillator I2,

The characteristics of a pentode are such that with a sinusoidal voltage of frequency f impressed upon the suppressor grid the plate current variations are non-sinusoidal and hence contain a D.C. component. In addition when the same frequency f but of much lower magnitude is irnpressed upon the control grid, then the magnitude of the D.C. component mentioned above will depend upon the amplitude of that signal which is impressed upon the control grid. When the maximum amplitude of the signal voltage impressed upon the grid is small compared to the grid bias then the D.C. component of conduction current will vary linearly with the amplitude of the signal on the grid. Variable attenuator 2| will provide means for controllingT the amplitude of the signal upon the grid.

If the beat frequency between the two injected frequencies is zero, then the D.C. output of the mixer will be proportional to the amplitude of the R.F. power lost. To correct for the phase shift through the stages and to insure zero beat, phase shifter is provided. Phase shifter is therefore adjusted until meter 44 stops oscillating.

Since one type of D.C. meter in common use will respond only to frequencies of the order one cycle per second, the R.C. network associated with the meter insures a long period response. In other words by proper choice of elements in the R.C. network the effective bandwidth of the D.C. metering circuit may be made as low as one audio frequency cycle.

The response of the lock-in amplifier will be linear in D.C. voltage output if the signal input be small compared to the D.C. bias on the mixer and the bolometer response is linear. Therefore in order to measure the small changes in conduction current eiiectively, a microammeter must be used. In using such a sensitive meter, noise originating in the amplier circuits may overload the mixer. To prevent this, network 28 is inserted between the plate and grid in one stage of the audio amplifier to by-pass every frequency except the particular audio frequency involved.

For simple standing wave measurement Littlefuse element I6 may be removed. The balanced bridge is no longer needed and the Littlefuse probe |5 with the direct current supply B plus connected thereacross is coupled to the audio amplier 20, The standing wave ratio can then be calculated from the maximum and minimum meter readings as probe I5 is moved along the line.

The radiation pattern may be obtained by plotting the relative field intensities as probe l5 is moved at a constant distance from a radiating element connected to output terminal I3.

It is pointed out that the lock-in amplifier described herein is but one of several circuits which may be used. It is obvious that any frequency converter with a D.C. output meter that is properly designed will serve the purpose for which this lock-in amplifier was used.

In using a sensitive D.C. current meter there must be means provided for making the meter zero independent of changes in line voltage. By choosing the proper resistances in the circuit this condition is quite well satisfied. As the plate voltage, for example, rises the audio oscillator oscillates more strongly causing the mixer plate current to rise by just the proper amount to balance the change in D.C. bias, thus providing automatic compensation.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as set forth in the appended claims.

The invention claimed is:

1. Radio frequency measuring apparatus comprising a means for generating a radio frequency wave, a means for modulating said radio frequency wave with an audio frequency, a transmission line, means to apply the modulated radio frequency wave as an input signal to said line,

- a first and a second bolometric resistive element, means to couple said rst and second resistive elements to said line at predetermined points thereon, means to impress a constant D..C. potential across said resistive elements, the resistance and voltage drop across each resistive element lbeing proportional to the instantaneous amplitude of the modulated radio frequency, means whereby the instantaneous voltage across the resistive ele-ments are subtracted to'produce an output voltage, the fundamental component of which is equal to the modulation frequency, an electron discharge device having a control electrode, a means for injecting said output voltage into said control electrode, said electron discharge device also containing an anode, a suppresser electrode, a screen electrode, and a cathode, a means for injecting a signal from said modulating means directly into said suppressor electrode, and a meter to measure the D.C. component of plate current flow in said device.

2. Radio frequency measuring apparatus comprising means for generating a radio frequency Wave, means for modulating said radio frequency wave with an audio frequency, a transmission line, means to introduce said modulated radio frequency wave as an input signal to said line, an electron discharge device including two grids and output means, means for applying said audio frequency to one of said grids, a bolometer bridge f including a pair of bolometric resistive elements coupled to said line at predetermined points thereon, the output of said bridge containing an audio frequency component being connected to the other` of said grids, and means for measuring the direct current component of the output of said electron discharge device.

3. Radio frequency measuring apparatus comprising a means for generating a radio frequency wave, a source of an audio frequency wave, a means for modulating said radio frequency wave with said audio frequency wave, a means for detecting a portion of the audio frequency component of said radio frequency wave, an electron discharge device having at least two grid elements, a means for injecting the detected component into one of said grids, a means for injecting said audio frequency wave directly from said audio frequency source into the other of said grids, and a means associated with said electron discharge device for measuring the direct current variations through the electron discharge device.

4. Radio frequency measuring apparatus comprising means for generating a radio frequency .A

wave, means for generating a modulation wave to modulate said radio frequency Wave, a transmission line, means to introduce said modulated radio frequency wave as an input signal to said transmission line, a bolometric resistive element coupled to said transmission line at a predetermined point thereon to detect the modulation component of said modulated radio frequency Wave, a frequency converter, means coupling said modulation Wave generating means and said modulation component detecting element to said frequency converter to derive a direct current component in the output thereof in accordance with the amplitude of said modulation component, and means for measuring the direct current component in the output of said frequency converter.

5. Radio frequency measuring apparatus comprising means for generating a radio frequency Wave, means for generating a modulation wave to. modulate said radio frequency wave, a transd missionv line, means to introduce said modulated radio frequency wave as an input signal to said transmission line, a bolometric resistive element coupled to said transmission line at a predetermined point thereon to detect the modulation component of said modulated radioV frequency wave, an electron discharge device having at least cathode, plate and two grid electrodes, means coupling said modulation component detecting element to one of said grids, means coupling said modulation wave generating means to the other of said grids, and means for measuring the direct current component of plate current in said discharge device to determine the amplitude of said modulation component.

6. The combination, as defined in claim 1, wherein said first and second bolometric resistive elements are coupled to said transmission line at the input and output thereof, respectively, whereby the amplitude of the fundamental component of said output voltage is proportional to the power loss of said transmission line.

'7. An instantaneous radio frequency power measuring device comprising a source of power at a first frequency, an audio frequency source of power at a second frequency coupled to said first frequency source to modulate the output thereof, a medium for` transmitting the modulated first frequency output, means coupled to said medium at at least one predetermined point for detecting an audio frequency component of said modulated radio frequency wave proportional in amplitude to the instantaneous power at said first frequency at said point, means coupled to said Vaudio frequency source and said detecting means for producing a direct current proportional in amplitude to said instantaneous power at said rst frequency at said point, and a direct current meter coupled to said direct current producing means for indicating said instantaneous power at said first frequency at said point.

8. A device according to claim 7 wherein said detecting means comprises a pair of devices coupling said voltage generating means to said medium at a pair of spaced points and means for generating a voltage proportional to the difference in the instantaneous power of said Yfirst frequency at said two points whereby said direct current meter indicates the power loss between said two points.

9. A device according to claim '7 wherein said detecting means comprises a bolometric bridge.

10. A device according to claim 7 wherein said direct current producing means comprises an electron discharge device including at least a cathode, a control grid, a suppressor grid, and a plate, said second frequency source being coupled to said suppressor grid and said detecting means being coupled to said control grid.

l1. A device according to claim '7 further comprising a damping circuit coupled between said direct current producing means and said meter, said damping circuit being designed to pass substantially only direct current and currents having frequencies up to one cycle per second waves.

12. An instantaneous radio frequency power measuring device comprising a source of radio frequency power, an audio frequency source coupled to said radio frequency source to amplitude modulate the output from said radio frequency source, a medium coupled to said radio frequency source for translating the modulated radio frequency output, a network coupled to said medium at at least one predetermined point foi` 7 8 detecting an audio frequency component of said REFERENCES CITED modulated radio frequency Wave proportional in The following references are of record in the amplitude to the instantaneous radio frequency me of this patent: power at said point. a frequency converter coupled to both said audio frequency source and said net- 5 UNITED STATES PATENTS Work for producing a. direct current proportional Number Name Date in amplitude to said instantaneous radio fre- 1,869,209 Mead July 26, 1932 quency power at said point, and a, direct current 2,221,115 Shepard Nov. 12, 1940 meter coupled to said frequency converter for 2,223,840 Wolff Dec. 3, 1940 indicating the instantaneous radio frequency 10 2,270,243 Bach Jan. 20, 1942 power at said point.

ROBERT H. DICKE. 

